Phase and frequency modulator circuits



Oct. 26, 1965 D. J. COMEZR PHASE AND FREQUENCY MODULATOR CIRCUITS 2Sheets-Sheet 1 Filed Aug. 28, 1962 PRIOR ART PHASE SHIFT (9) REQISTANCETHYRITE FIG.6

IN VENTOR.

DAVID J. COMER AGENT Oct. 26, 1965 D. J. COMER PHASE AND FREQUENCYMODULATOR CIRCUITS 2 Sheets-Sheet 2 Filed Aug. 28, 1962 HIGH FREQUENCYOCILLATOR MODULATION FIG.7

FIG. 8

3,214,710 PHASE AND FREQUENCY MODULATOR CTRCUITS David J. Corner, SanJose, Calif, assignor to International Business Machines Corporation,New York, N.Y., a

corporation of New York Filed Aug. 28, 1962, er. No. 219,945 3 Claims.(Cl. 33229) This invention relates to modulation circuits and moreparticularly to circuits for phase and frequency modulating a carriersignal.

Of the several relatively simple phase modulation circuits in use at thepresent time, a limitation common to all of them is that the modulationis linear only if the degree of carrier phase shift never exceeds acertain value. This value for simple circuits lies between and degrees.More complex circuits are required to extend the linear range of phaseshift up to approximately 60 degrees.

Frequency modulators, in the form of phase controlled variable frequencyoscillators, have much the same problem as those above mentioned for thephase modulators. That is, linear frequency deviations available forchanges in modulating voltage are limited unless the phase controllersare made quite complex to achieve the necessary wide linear phasevariation.

Accordingly, it is an object of this invention to provide an improvedphase modulator.

It is another object of this invention to provide an improved phasemodulator of simple design.

Still another object of this invention is to provide an improved phasemodulator which maintains simplicity of design while providing asubstantially linear wide range phase shift.

It is also an object of this invention to provide an improved frequencymodulator which exhibits wide range linear changes in frequency forchanges in applied modulating signal.

The above stated objects are attained by first applying to a phasesplitter a relatively high frequency carrier signal superimposed upon anamplitude varying modulating signal. The phase splitter produces twosignal outputs, one of which is displaced by 180 from the other. One ofthe signal outputs is fed through a reactive means where it experiencesapproximately a 90 phase shift. The outher output signal is fed througha resistive device whose resistance varies in an inversely relatedsubstantially hyperbolic manner in response to changes in voltageapplied thereacross. When the outputs from the resistive means andreactance means are combined, it can be shown that practically a linearphase shift is obtained. Basically, this is so because as the resistanceof the resistance means varies in accordance with voltage levelvariations of the carrier signal, the non-linear resistance changesbalance out the non-linearities of the phase shift circuit.

In a frequency modulation embodiment of this invention, phase modulationmeans of the type described above, which normally exhibit a phase shiftof 49, is provided with a feedback having a phase shift of 360-0. Thecircuit then oscillates at a basic frequency which establishes a 360phase shift around the the circuit loop. When the phase shift throughthe phase modulation means is changed by the application of a modulatingsignal, the frequency of oscillation automatically changes to maintainthe required 360 loop phase shift.

The foregoing and other objects, features and advan tages of theinvention will be apparent from the following more particulardescription of preferred embodi- United States Patent 0 3,214,710Patented Oct. 26, 1965 ments of the invention, as illustrated in theaccompanying drawings.

In the drawings:

FIG. 1 is a circuit diagram of a prior art phase shifter.

FIG. 2 is a vector diagram showing relative voltage relationships in thecircuit of FIG. 1.

FIG. 3 is a graph showing the variation of circuit phase shift withvariations in resistance in the crcuit of FIG. 1.

FIG. 4 is a graph depicting the characteristic variations of Thyriteresistance with variations in voltage applied thereto.

FIG. 5 is a circuit diagram of an embodiment of the invention.

FIG. 6 is a diagram of signals appearing in the circuit of FIG. 5.

FIG. 7 is a circuit diagram of an embodiment of the invention whereintransistor circuitry is utilized.

FIG. 8 is a circuit diagram of a frequency modulation circuit whichembodies the subject invention.

In order to gain a better understanding of the subject invention, it isfirst desirable to analyze the prior art split-phase phase shifter.shown in FIG. 1. Oscillator 12. is connected to the primary winding 13of transformer 14. The secondary windings 16 and 18 of transformer 14are center-tapped to ground and thereby produe phase-opposed voltageoutputs. Connected to the windings 16 and 18 are variable resistor :20and capacitor 22, respectively. The output from this phase shift circuitis taken between terminal 24 and ground.

In FIG. 2 there is shown a vector diagram which depicts therelationships between the various voltages and phases found in the phaseshifter shown in FIG. 1. The voltages appearing across the center tappedsecondary windings 16 and 18 of transformer 14 are phase opposed and arerespectively represented as V and V V is the voltage drop acrossresistor 20 and Vcgg is the voltage drop across capacitor 22. Since, byKirchhotfs law, the sum of the voltage drops around the circuit mustequal zero, the voltage vector diagram is a closed triangle and theoutput voltage at terminal 24 is represented by a vector drawn from theground connection between V and V and the right angle intersectionbetween V and V It can be shown that the phase shift 6 of the outputvoltage V (the included angle between V and V is twice the includedangle between V and V With these relationships in mind, the followingequations can be derived from the diagram:

From Equation (2) it can be seen that the phase shift 0 of the outputvoltage V varies as an inverse tangential function of changes in eithercapacitor 2?. or resistor 20 (assuming all frequencies constant). A plotof Equation (2) showing the variation of phase shift 6 as the resistanceof R is varied as shown in FIG. 3 by curve 26. Since curve 26 clearlyresembles a. hyperbola, it can be approximated by an even simplerexpression than Equation (2), i.e.,

6 where K; is a constant 3 From Equation (3) and FIG. 3 it is obviousthat for any extended variations of resistance, the phase shift of thecircuit shown in FIG. 1 is far from linear.

Assume now that a device is substituted for variable resistor 20 whosevariations of resistance are substantially hyperbolic in relation tochanges in voltage applied thereacross. A material which experiences theaforementioned resistance variations is Thyrite which is described inUS. Patent 1,822,742 to McEachron. Another device which exhibits thischaracteristic is a properly biased semiconductor diode. The diode isnot so satisfactory as the Thyrite due to the very limited voltage rangeover which it exhibits the necessary characteristic response.

The hyperbolic resistance voltage relationship curve 28 as shown in FIG.2 of the McEachron patent is reproduced in the drawings as FIG. 4. Sincecurve 28 clearly resembles a hyperbola, the relationship between thevoltage across the Thyrite V and the resistance of the Thyrite, R can beexpressed as:

R Where K is constant Equation (5) illustrates that the phase shiftthrough a circuit of the type shown in FIG. 1 (where a Thyrite resistorhas been substituted for variable resistor 20) will be a substantiallylinear function of the voltage across the Thyrite. This relationshipwill hold true so long as the approximations made in the derivations ofEquations (3) and (4) hold. From the practical standpoint, theseapproximations are valid for circuit phase shifts up to 60.

With reference now to FIG. 5, a phase modulation circuit is shown whichutilizes the Tryrite phase shift circuit described above. The primarywinding 32 of transformer 34 is energized by a carrier oscillator 36. Tothe center tap between secondary windings 38 and 40, there is applied asource of modulating signals 42 and the output from bias battery 44.Thyrite resistor 46 is connected to winding 38 and forms one-half of thephase modulation circuit. The DC. current supplied by battery 44 acts toestablish the operating point of Thyrite resistor 46. Capacitor 48,which is connected to secondary winding 40, forms the other half of thebasic phase modulation circuit (an inductor could also be used). Outputconductor 50 which is connected between Thyrite resistor 46 andcapacitor 48 also forms a common connection point for tuned circuit 51which includes capacitor 52 and inductor 54. Tuned circuit 51 is tunedto resonate and thereby provide its highest impedance at the outputfrequency of carrier oscillator 36.

The signals which appear across secondary windings 38 and 44) areillustrated in FIG. 6. Modulating signal 62 (a relatively low frequency)is produced by modulating signal source 42 and applied to the center tapbetween windings 38 and 40. Carrier signal 64 which is induced insecondary windings 38 and 40 by primary winding 32 is a high frequencyoscillation generated by carrier oscillator 36.

The resultant output from secondary windings 38 and 40 is shown by waveform 66. From this it can be seen that a superimposition occurs wherebythe level of carrier signal 64 is made to vary in accordance with thevoltage amplitude variations of modulating signal 62.

In FIG. 5 the superimposed carrier signal 66 is applied to Thyriteresistor 46 and, phase displaced by 180, to capacitor 48. Since tunedcircuit 51 is resonant at the carrier signal frequency, it presents alarge impedance thereto and a small impedance to all other frequencies,i.e., the modulating signal frequency. Additionally, op-

4 crating resistances of Thyrite fall substantially in the range of1-3OK ohms, whereas the impedance of tuned circuit 51 to carrier signal64 may be made many times these values. Thus, practically the entirevoltage drop due to carrier signal 64 appears across tuned circuit 51while substantially the entire voltage drop due to modulaing signal 62appears across Thyrite resistor 46 (the tuned circuit 51 being a lowimpedance to ground at this frequency). The impedance of capacitor 48remains substantially constant throughout the operation of the circuit.

It should now be apparent that the phase displacement of the carriersignal by the phase shifting network at output terminal 5% dependsalmost entirely upon the voltage level of the modulating signal 62. Thus(see FIG. 2) as the modulating signal increases in voltage amplitude,the resistance of the Thyrite decreases, thereby causing the outputvoltage vector to rotate counterclockwise, increasing the phase shift 0.Conversely, as the amplitude of the modulating voltage decreases, theresistance of the Thyrite increases and the phase shift is caused todecrease. There is therefore achieved a very simple phase modulatorwhich provides wide linear phase variations in accordance with amplitudevariations of a modulating signal.

If it is desired to utilize a carrier signal whose amplitude is small inrelation to the amplitude variations of the modulating signal, tunedcircuit 51 can be eliminated. This is so because the voltage drop acrossThyrite resistor 46 will still be substantially controlled by theamplitude variations of the modulating signal and will be littleaffected by the carrier signal.

In FIG. 7, there is shown an embodiment of the invention wherein atransistor phase splitter is utilized in lieu of the transformer circuitshown in FIG. 5. In this circuit, the carrier signal is superimposedupon the modulating signal and applied to base of transistor 72. As iswell known, the voltage which appears across collector resistor 74 willbe phase displaced by 180 from the voltage appearing across emitterresistor 76. There is thereby supplied to the Thyrite-capacitor phaseshift circuit, the required split-phase superimposed carrier signal.

The phase modulated outputs from the circuits of FIGS. 5 and 7 will havelittle, if any, amplitude variations. This is due to the fact that theoutput conductor is essentially at ground potential with respect to thelow frequency modulating signal. Therefore, the potential appearing atthe output is essentially that produced by the phase shifted carriersignal (a constant amplitude signal) appearing across the tuned circuit.

With reference now to FIG. 8, there is shown a circuit diagram of afrequency modulator which employs the circuit shown in FIG. 7.Modulating signal source 82 applies its output through capacitor 84 tophase shift circuit and through capacitor 88 to phase shift circuit 81.Phase shift circuits 89 and 81, which respectively include transistors86 and 90 and their associated circuitry, are substantially identical tothe phase modulator shown in FIG. 7. For simplicity, the base electrodebiasing circuitry is not shown in FIG. 8. The only difference betweenthe phase shift circuits of FIG. 7 and FIG. 8 is that tuned circuits 91and 93 in each of phase shift circuits 80 and 81 have been shunted byresistors 92 and 94. These resistors are included to lower the Q ofthese tuned circuits for a purpose to 'be hereinafter described.

Phase shift circuit 80 is connected through buffer amplifier 96 to thebase of transistor 96 in phase shift circuit 81. The output from phaseshift circuit 81 is applied through bufi'er amplifier 98 to aconventional amplifier 100 which introduces a 180 phase shift into areceived signal. Buffer amplifiers 96 and 98 are preferably emitterfollower or cathode follower amplifiers which isolate and impedancematch the various circuits while introducing no phase shift into asignal.

The output from amplifier is fed back via conductor 102 through variableresistor 104 to the base cirsuit of transistor 86 in phase shift circuit80. As will hereinafter be seen, the result of this feedback is that thecircuit of FIG. 8 oscillates at a frequency which is determined by theamount of phase shift introduced into the circuit loop by phase shiftcircuits 80 and 81. Since the oscillation frequencies of this circuitare much higher than the operational frequencies of modulating source'82, capacitors 84 and 88, which present small impedances to the lowerfrequency modulating signal, are included to substantially block theoscillator feedback on line 102 from entering either modulating source82 or the base circuit of transistor 90.

Regarding the operation of the frequency modulator, reference should befirst made to the operation of phase shift circuits 80 and 81. Aspreviously described, the phase shift through a circuit such as is shownin FIG. 7 can be expressed as:

Equation (6) indicates that any variation in (R or in ((tan results inan inverse variation of the frequency of a signal passing through thephase shifter. But, if oscillations are to be sustained in the frequencymodulator, two well-known requirements must be met, i.e., a loop gain ofone or greater and a loop phase shift of 360. Since amplifier 100introduces a 180 phase shift into the signal, it follows that phaseshift circuits 80 and 81 must each introduce a phase shift of 90 for therequired loop phase shift of 360 to be satisfied. Therefore, so long asthe loop gain exceeds one, oscillations will automatically be sustainedwith the circuit phase shifts establishing themselves so as toaccomplish the required 360 loop phase shift.

With the above facts in mind, Equation (6) can be simplified even more.When the circuit is oscillating, the phase shift through each of phaseshift circuits 380 and 81 is 90 and the tangent of 0/2 or 45=1. Thefrequency f as defined in Equation (7) can then be expressed as afunction of the modulating voltage:

therefore, f

therefore, f= K V (where K =a constant) (9) 2 Referring to FIG. -2, itis observed that the phase shift 0 equals only when V =V or in otherwords, when the resistance and reactance in the respective circuit armsare equal. Thus, the frequency of oscillation of the frequency modulatorwill be that frequency where the reactances of capacitors 106 and 108are respectively equal to the resistances of Thyrite resistors 110 and112.

With regard now to the operation of the frequency modulator in FIG. 8,assume that the circuit is oscillating. If the voltage output frommodulating source '82 becomes more negative, the conduction intransistors 86 and 90 increases. This results in increases in theVoltage drops across Thyrite resistors 110 and 112, re spectively, withattendant decreases in their resistance. This action produces anincrease in phase shift through each of phase shift circuits 80 and 81.Since the gain of the loop is adjusted to be at least one or greaterthrough the combination of amplifier and variable resistor 104, thefrequency of oscillation in the circuit increases to equalize thereactive voltage drops across capacitors 106 and 108 with the decreasedresistive voltage drops across Thyrite resistors 110 and 112,respectively. Thus, the new frequency of oscillation is that at whicheach phase shifter produces a 90 shift with an accompanying loop phaseshift of 360.

It can now be seen that if the voltage output from modulating source 82increases, the result is an increase in the Thyrite resistances due tothe decrease in voltages thereacross. This causes a correspondinglessening of the phase shift through each of phase shift circuits 80 and81. In response, the frequency of oscillation of the frequency modulatordecreases thereby maintaining the required 360 loop phase shift.

As afore-stated, circuits 91 and 93 are low Q tuned circuits, that is,their resonance curves are relatively flat over a wide range offrequencies instead of being peaked at a single or small group offrequencies (high Q). Proper design therefore results in theirpresenting unifor-mly high impcdances to all expected oscillationfrequencies. Thus, as in FIGS. 5 and 7, the low frequency modulationvoltages appear substantially across the Thyrite resistors whereas theoscillation frequency voltages appear substantially across the tunedcircuits. This action prevents the oscillation voltage frequencies fromaffecting the Thyrite resistance values.

It should be appreciated that only one phase shift circuit could be usedin the frequency modulator (instead of the two shown in FIG. 8). Thiswould present certain problems however. One phase shift circuit couldaccomplish a phase shift 0, but an additional phase shift of 360-0 wouldhave to be introduced into the loop for oscillations to be sustained.For example, if it is assumed that 9=90, then 3606=270. If an amplifierwere used to shift the phase 180 (as is shown in FIG. 8) then theadditional phase shift needed in order that the circuit oscillate wouldhe 270-180=90. Networks are available which will provide this phaseshift with a substantially constant attenuation at all oscillationfrequencies, but they are complex. By utilizing a pair of phaseshifters, as in FIG. 8, any requirement for such complex circuitry iseliminated.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it Will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

I claim:

1. In a phase modulator to which there is fed a high frequency carriersignal superimposed upon an amplitude varying modulating signal, thecombination comprising:

a phase splitter having first and second outputs, both said out-putsproducing said superimposed signals, the signals emanating from saidfirst output phase displaced substantially 180 from the signalsemanating from said second output;

variable resistance means connected to said first output,

the resistance of said variable resistance means exhibiting an inverselyrelated substantially hyperbolic variation in relation to a variation inthe voltage applied thereto;

reactive means connected to said second output for shifting the phase ofsaid superimposed signals emanating from said second output bysubstantially 90; and

output means connected between said variable resistance means and saidreactive means, said output means producing said high frequency carriersignal having phase displacements proportional to amplitude variationsof said modulating signal.

2. The invention as recited in claim 1 with the further provision oftuned circuit means connected to said output means, said tuned circuitmeans exhibiting its highest impedance at the frequency of said carriersignal.

3. A circuit for varying the phase of a carrier signal in accordancewith amplitude variations of a modulating signal which includes a sourceof high frequency carrier signals superimposed upon an amplitude varyingmodulating signal, the combination comprising:

phase splitting means fed from said source having first and secondoutputs, both said outputs producing said superimposed signals, thesignals emanating from said first output phase displaced substantially180 from the signals emanating from said second output;

capacitive reactance means connected to said first output for shiftingsaid superimposed signals emanating from said output by substantially 90in phase;

non-linear resistance means connected to said second output, theresistance of said non-linear resistance means exhibiting an inverselyrelated substantially hyperbolic variation in relation to a voltageapplied thereto; and

tuned circuit means connected between said non-linear resistance meansand said capacitive reactive means, said tuned circuit means exhibitingits highest impedance at the frequency of said carrier signal.

4. In a phase modulator to which there is fed a superimposed signalwhich includes a high frequency carrier signal superimposed upon anamplitude varying modulating signal V, the combination comprising:

a phase splitter having first and second outputs, both said outputsproducing said superimposed signal, the signals emanating from saidfirst output phase displaced substantially 180 from the signalsemanating from said second output;

variable resistance means connected to said first output, the resistanceR of said resistance means varying in response to amplitude variationsof said superimposed signal substantially in accordance with theexpression where K is a predetermined constant;

reactive means connected to said second output for shifting the phase ofsaid superimposed signals emanating from said second output bysubstantially 90;and

output means connected between said variable resistance means and saidreactive means, said high frequency carrier signal emanating from saidoutput means having a phase displacement 0 expressed by the function 9=KV where K is a predetermined constant.

5. In a frequency modulation circuit, the combination comprising:

phase shift means providing a normal phase shift 0,

said phase shift means including: a phase splitter having an inputadapted to have applied thereto an amplitude varying modulating signal,said phase splitter having in-phase and out-of-phase outputs; non-linearresistance means connected to one of said outputs, said non-linearresistance means exhibiting inversely related substantially hyperbolicvariations in resistance in relation to changes in said modulatingsignal; reactive means connected to another output from said phasesplitter, said reactive means being adapted to shift the phase of asignal by substantially and output means connected between saidnon-linear resistance means and said reactance means, said phase shiftmeans exhibiting substantially linear changes in said phase shift 0 inaccordance with changes in said modulatingsignal; and

means exhibiting a phase shift of 3600 connected between said outputmeans and said phase splitter, said means including amplification means;whereby said circuit oscillates at frequencies which are dependent uponvariations in amplitude of said modulating signal.

6. The invention as recited in claim 5 wherein said output meanscomprises a low Q tuned circuit exhibiting substantially constantimpedances over the range of frequencies of oscillation of saidfrequency modulator.

7. In a frequency modulation circuit to which there is applied anamplitude varying modulating signal, the combination comprising:

first and second series connected phase shift means,

each said phase shift means providing a normal phase shift, 0, each saidphase shift means including: a phase splitter to which said amplitudevarying modulating signal is applied, said phase splitter havingin-phase and out-of-phase outputs; non-linear resistance means connectedto one of said outputs, said non-linear resistance means exhibitinginversely related substantially hyperbolic variations in resistance inrelation to changes in said modulating signal; reactive means connectedto another output from said phase splitter, said reactive means beingadapted to shift the phase of a signal by substantially 90; and outputmeans connected between said nonlinear resistance means and saidreactance means, each said phase shift means exhibiting substantiallylinear changes in said normal phase shift 0 in accordance with changesin said modulating signal; and

feedback means exhibiting a constant phase shift of 36020 connectedbetween the output means of said second phase shift means and the phasesplitter in said first phase shift means, said feedback means includingamplification means; whereby an oscillating circuit is formed whosefrequencies of oscillation are dependent upon variations in saidmodulating signal.

8. The invention as recited in claim 7 wherein said output meanscomprises a low Q tuned circuit exhibiting substantially constantimpedances over the range of frequencies of oscillation of saidfrequency modulator.

References Cited by the Examiner UNITED STATES PATENTS 1,950,406 3/34Hoorn 332--23 2,284,401 5/42 Manley et al. 33229 X 2,790,147 4/57Armstrong et al 33223 X ROY LAKE, Primary Examiner.

ALFRED L, BRODY, Examiner.

1. IN A PHASE MODULATOR TO WHICH THERE IS FED A HIGH FREQUENCY CARRIERSIGNAL SUPERIMPOSED UPON AN AMPLITUDE VARYING MODULATING SIGNAL, THECOMBINATION COMPRISING: A PHASE SPLITTER HAVING FIRST AND SECONDOUTPUTS, BOTH SAID OUTPUTS PRODUCING SAID SUPERIMPOSED SIGNALS, THESIGNALS EMANATING FROM SAID FIRST OUTPUT PHASE DISPLACED SUBSTANTIALLY180* FROM THE SIGNALS EMANATING FROM SAID SECOND OUTPUT; VARIABLERESISTANCE MEANS CONNECTED TO SAID FIRST OUTPUT, THE RESISTANCE OF SAIDVARIABLE RESISTANCE MEANS EXHIBITING AN INVERSELY RELATED SUBSTANTIALLYHYPERBOLIC VARIATION IN RELATION TO A VARIATION IN THE VOLTAGE APPLIEDTHERETO; REACTIVE MEANS CONNECTED TO SAID SECOND OUTPUT FOR SHIFTING THEPHASE OF SAID SUPERIMPOSED SIGNALS EMANATING FROM SAID SECOND OUTPUT BYSUBSTANTIALLY 90*; AND OUTPUT MEANS CONNECTED BETWEEN SAID VARIABLERESISTANCE MEANS AND SAID REACTIVE MEANS, SAID OUTPUT MEANS PRODUCINGSAID HIGH FREQUENCY CARRIER SIGNAL HAVING PHASE DISPLACEMENTSPROPORTIONAL TO AMPLITUDE VARIATIONS OF SAID MODULATING SIGNAL.